1. Field of the Invention
The present invention relates to a nonreciprocal circuit device such as an isolator or a circulator used in a high frequency band, for example, in a microwave band. In addition, the invention relates to a communication apparatus incorporating the nonreciprocal circuit device.
2. Description of the Related Art
In conventional nonreciprocal circuit devices such as lumped-constant isolators and circulators, attenuation in a signal-propagating direction is extremely small, whereas attenuation in the opposite direction is extremely great. Thus, the conventional nonreciprocal circuit devices having such characteristics are widely used in communication apparatuses to allow oscillators and amplifiers to act in a stable manner while maintaining their functions.
FIG. 19 shows an exploded perspective view of a conventional isolator, and each of FIGS. 20A and 20B shows the inner structure of the isolator. FIG. 21 shows an equivalent circuit diagram of the isolator.
As shown in FIG. 19 and FIGS. 20A and 20B, in the lumped-constant isolator, a magnetic assembly 5 composed of a ferrite member 54 and central conductors 51, 52, and 53, a permanent magnet 3, and a resin frame 7 are arranged in a magnetic closed circuit composed of an upper yoke 2 and a lower yoke 8. In the resin frame 7, port P1 of the central conductor 51 is connected to an input/output terminal 71 and a matching capacitor C1. Port P2 of the central conductor 52 is connected to an input/output terminal 72 and a matching capacitor C2. Port P3 of the central conductor 53 is connected to a matching capacitor C3 and a termination resistor R. One end of each of the capacitors C1, C2, and C3 and one end of the termination resistor R are connected to grounds 73.
In the equivalent circuit shown in FIG. 21, the ferrite member has a disk-like shape and a DC magnetic field is indicated by the symbol H. The central conductors 51, 52, and 53 are shown as equivalent inductors L. In such a circuit structure, forward-direction characteristics are equivalent to the characteristics of a band pass filter. In frequency bands distant from the pass band, even in the forward direction, signals are slightly attenuated.
In general, in a conventional communication apparatus, an amplifier used in a circuit of the apparatus usually causes some distortions. This is a factor producing spurious components such as the second and third harmonics of a fundamental frequency, by which unnecessary radiation is generated. Since such unnecessary radiation emitted from the communication apparatus causes the malfunction of a power amplifier and a problem of interference, standards and specifications are determined in advance to suppress the unnecessary radiation below a certain level. In order to prevent the unnecessary radiation, it is effective to use an amplifier having good linearity. However, since such an amplifier costs much, for example, a filter is usually used to attenuate unnecessary frequency components. Still, such a filter is expensive and the size of the apparatus increases. In addition, there is a loss generated by the filter.
Thus, it is considered that spurious components can be suppressed by using the characteristics of a band pass filter included in an isolator or a circulator. However, it is impossible to obtain sufficient attenuation characteristics in unnecessary frequency bands by using the conventional nonreciprocal circuit device having a basic structure shown in each of FIGS. 19 to 21.
In order to solve the above problems to obtain a large amount of attenuation in spurious frequency bands such as the second and third harmonics of a fundamental frequency, there is disclosed a nonreciprocal circuit device in Japanese Unexamined Patent Application Publication No. 10-93308. Each of FIG. 22, FIGS. 23A and 23B, and FIG. 24 shows an isolator as an example of the nonreciprocal circuit device. FIG. 22 shows an exploded perspective view of the isolator. Each of FIGS. 23A and 23B shows the inner structure of the isolator. FIG. 24 shows an equivalent circuit diagram of the isolator.
Unlike the isolator shown in each of FIGS. 19 to 21, this isolator includes an inductor Lf for a band pass filter. The inductor Lf is connected between port P1 of a central conductor 51, a matching capacitor C1, and an input/output terminal 71. As the inductor Lf, a solenoid coil is used, which is adaptable to miniaturization of the circuit structure. An isolator applied in the 1 GHz band uses a coil having an inductance of approximately 24 nH. More specifically, the used coil is formed by making nine turns of a copper wire having a width xcfx86 of 0.1 mm with an outside diameter xcfx86 of 0.8 mm.
A capacitor Cf is connected in series to the input/output terminal 71 of the isolator having the above structure. With this arrangement, as seen in the equivalent circuit diagram shown in FIG. 24, the capacitor Cf and the inductor Lf form a band pass filter. As a result, the signal components of frequencies distant from the pass band can be attenuated.
FIG. 25 shows a graph illustrating frequency characteristics of the isolator (a first conventional example) shown in FIGS. 19 to 21 and the isolator (a second conventional example) shown in FIGS. 22 to 24. This graph shows the frequency characteristics of the isolators applied in the 1-GHz band. When a comparison is made between the first conventional isolator and the second conventional isolator, it is found that attenuation of the second harmonic (2 GHz) is increased from 20.2 dB to 33.3 dB, and attenuation of the third harmonic (3 GHz) is increased from 28.2 dB to 46.4 dB.
Thus, when the solenoid coil is arranged in the nonreciprocal circuit device to form a filter attenuating unnecessary frequency components, the entire circuit structure can be made smaller than the structure including a discrete filter disposed outside of the device.
Recently, with an increasing need for further miniaturization of a mobile communication apparatus, there has been a demand for a smaller nonreciprocal circuit device incorporating an inductor for a filter. Thus, it is also necessary to reduce the size of the inductor for a filter. However, when an inductor formed by a solenoid is miniaturized, inductance of the inductor becomes smaller, thereby reducing attenuation in the second and third harmonics of the fundamental frequency. In addition, in order to miniaturize such a solenoid inductor without causing inductance reduction, it is considerable to form a solenoid inside a magnetic member. However, this arrangement requires a magnetic member and such a structure is difficult to manufacture, increasing cost.
Accordingly, it is an object of the present invention to provide a compact nonreciprocal circuit device in which a large amount of attenuation can be obtained in a predetermined frequency band without increasing cost. It is another object of the invention to provide a communication apparatus using the nonreciprocal circuit device.
According to a first aspect of the invention, there is provided a nonreciprocal circuit device including a magnetic member to which a DC magnetic field is applied, the magnetic member including a plurality of central conductors arranged to mutually intersect, one end of each of the central conductors being grounded, and a plurality of matching capacitors connected to a non-grounded end of each of the central conductors, in which at least one of the matching capacitors has a self-resonance frequency equal to or less than four times the central frequency of a pass band of the nonreciprocal circuit device.
In general, in a nonreciprocal circuit device, parallel resonance circuits are formed by central conductors having inductance components and matching capacitors to obtain matching with the central frequency of a pass band. With this arrangement, attenuation near the central frequency of the pass band can be almost removed. However, in this arrangement, it is impossible to obtain a filtering function for attenuating the spurious components of frequencies higher than the central frequency of the pass band. Thus, in the present invention, by appropriately designing the configurations of the matching capacitors, the self resonance frequency of the matching capacitor is set to be equal to or less than four times the central frequency of the pass band. Major spurious components are the second and third harmonics of a fundamental frequency (the central frequency of a pass band). The matching capacitor having a self-resonance frequency equal to or less than four times the central frequency acts as a trap filter for attenuating such spurious components. With this arrangement, the spurious components can be attenuated without increasing the number of components to be used.
Each of the capacitors used in the nonreciprocal circuit device may be a single-plate capacitor formed by disposing electrodes on both major surfaces of a dielectric substrate or a multi-layer capacitor formed by disposing electrodes on both major surfaces of a dielectric substrate and therein. In addition, each matching capacitor may be a chip capacitor formed by disposing a meandering-line electrode on a substrate. With such an arrangement, inductance components which the capacitor itself has can be increased and the capacitor can be made small having a low self-resonance frequency.
In addition, in the nonreciprocal circuit device, two or more matching capacitors may have self-resonance frequencies equal to or less than four times the central frequency of the pass band.
At least one of the matching capacitors may have a self-resonance frequency substantially two times the central frequency of the pass band.
At least one of the matching capacitors may have a self-resonance frequency substantially three times the central frequency of the pass band.
When the two or more matching capacitors have substantially the same self-resonance frequency equal to or less than four times the central frequency of the pass band, spurious components near the resonance frequency can be more significantly attenuated. Moreover, when the self-resonance frequencies of the two or more matching capacitors are different from each other, while both frequencies are equal to or less than four times the central frequency of the pass band, spurious components present over a wider frequency band can be attenuated. Major factors causing unnecessary radiation of a communication apparatus are spurious components such as the second and third harmonics of the fundamental frequency as mentioned above. Thus, by using the matching capacitor having the self-resonance frequency two times the fundamental frequency and the matching capacitor having the self-resonance frequency three times the fundamental frequency, the spurious components of the second and third harmonics of the fundamental frequency can be efficiently attenuated. In this case, the frequencies of xe2x80x9csubstantially two timesxe2x80x9d range from approximately 1.5 to 2.5 times the central frequency of the pass band. The frequencies of xe2x80x9csubstantially three timesxe2x80x9d range from approximately 2.5 to 3.5 times the central frequency of the pass band.
In the nonreciprocal circuit device according to the invention, the two or more matching capacitors may have self-resonance frequencies equal to or less than four times the central frequency of the pass band, and at least one of the matching capacitors may have a self-resonance frequency substantially twice the central frequency of the pass band.
Furthermore, at least one of the matching capacitors may have a self-resonance frequency substantially two times the central frequency of the pass band, and at least another matching capacitor may have a self-resonance frequency substantially three times the central frequency of the pass band.
In addition, the nonreciprocal circuit device may further include a series resonant circuit formed by connecting an inductor in series to the matching capacitor having the self-resonance frequency equal to or less than four times the central frequency of the pass band, the series resonant circuit having a resonance frequency over the central frequency of the pass band.
As shown here, when the inductor is connected in series to the matching capacitor, the resonance frequency of the series resonant circuit composed of the inductor and the matching capacitor becomes lower than the frequency four times the central frequency of the pass band as the self-resonance frequency of the matching capacitor. As a result, it is possible to form a trap filter by reducing the size of the matching capacitor. The self-resonance frequency of a matching capacitor to which no inductor is connected may be equal to or greater than four times the central frequency of the pass band or may be equal to or less than that.
Furthermore, the nonreciprocal circuit device may include series resonant circuits formed by connecting inductors to the two or more matching capacitors having the self-resonance frequencies equal to or less than four times the central frequency, the series resonant circuits having resonance frequencies over the central frequency of the pass band.
Furthermore, at least one of the series resonant circuits may have a resonance frequency substantially twice the central frequency of the pass band.
In addition, at least one of the series resonant circuits may have a resonance frequency substantially three times the central frequency of the pass band.
When the inductors are connected to the two or more matching capacitors to form the series resonant circuits, this arrangement can provide smaller trap filters capable of more greatly attenuating the spurious component of a specified frequency and attenuating spurious components existing over a wider frequency band. In this case, the frequencies of xe2x80x9csubstantially two timesxe2x80x9d range from approximately 1.5 to 2.5 times the central frequency of the pass band. The frequencies of xe2x80x9csubstantially three timesxe2x80x9d range from approximately 2.5 to 3.5 times the central frequency of the pass band.
The inductor described above is formed in various manners. For example, the inductor may be formed by extending one of the central conductors or may be formed by a chip to be disposed under a matching capacitor. In addition, the inductor may be formed either by integrating in the resin frame containing the matching capacitor or by cutting a part of a yoke forming a closed magnetic circuit. On upper and lower surfaces of chip inductors and chip capacitors, there can be disposed electrodes. Thus, by stacking these chip components, component space can be saved and connection between the components can be made easier. In addition, when forming the inductor by extending the central conductor, integrating the inductor in the resin frame, or by cutting a part of the yoke, the number of components to be used can be reduced. As a result, the manufacturing process can be simplified and cost can be reduced.
Furthermore, a series resonant circuit having substantially two times the central frequency of the pass band may be formed by connecting an inductor to at least one matching capacitor, and at least another matching capacitor may have a self-resonance frequency substantially three times the central frequency.
Furthermore, a series resonant circuit having substantially three times the central frequency of the pass band may be formed by connecting an inductor to at least one matching capacitor, and at least another one of the matching capacitors may have a self-resonance frequency substantially two times the central frequency.
Furthermore, a series resonant circuit having substantially two times the central frequency of the pass band may be formed by connecting an inductor to at least one matching capacitor, and another series resonant circuit may be formed by connecting an inductor to at least another matching capacitor having a self-resonance frequency substantially three times the central frequency.
Furthermore, an equivalent capacitance of each series resonant circuit at the central frequency of the pass band may be used as a matching capacitance with respect to the central frequency of the pass band. When the resonance frequency of the series resonant circuit is set to be higher than the central frequency of the pass band to remove the spurious components, the series resonant circuit exhibits a capacitive impedance with respect to the central frequency of the pass band. By appropriately setting the inductor and the capacitor of the series resonant circuit, there can be provided an equivalent matching capacitance with respect to the central frequency of the pass band. With this arrangement, when the series resonant circuit is disposed as a trap filter, it is unnecessary to dispose another matching capacitor. As a result, the number of components to be used can be reduced, thereby contributing to miniaturization of the device and cost reduction.
According to a second aspect of the present invention, there is provided a communication apparatus incorporating the nonreciprocal circuit device of the invention. In the communication apparatus, the nonreciprocal circuit device may be used as a circulator branching transmitted signals and received signals. With this arrangement, the communication apparatus can be miniaturized having good spurious characteristics.